1. Field of the Invention
The present invention generally relates to a variable attenuator, and more particularly to a variable attenuator formed on a semiconductor substrate or chip.
Recently, communication devices such as portable telephone sets have been designed to positively employ integrated circuits in order to meet the requirements for down sizing and lightening. Particularly, there is a need for an integrated circuit operable in the microwave band and a very high frequency band such as the milliwave band higher than 30 GHz. Such an integrated circuit has a circuit configuration capable of signals of very high frequencies. In many cases, such a circuit configuration is equipped with a configuration which attenuates the signals to a given level in order to prevent signals of excessive large levels from being applied to circuit components. Hence, there is a need for a variable attenuator which is simple and has high performance.
2. Description of the Related Art
FIG. 1 is a block diagram of a receive front end of a wireless device such as a portable telephone set. The receive front end includes an antenna 100, a band-pass filter 101, a low-noise amplifier 102, a variable attenuator 103, a frequency converter 104, a level detection diode 105 and a local oscillator 106.
A signal received via the antenna 101 is applied to the low-noise amplifier 102 via the band-pass filter 101 so that noise components contained therein can be eliminated. The low-noise amplifier 102 amplifies the fine received signal. The amplified output signal of the low-noise amplifier 102 s applied to the frequency converter 104 and the level detection diode 105 via the variable attenuator 103.
The level detection diode 105 outputs a detection signal PD indicating the level of the amplified signal. The detection signal PD is used to indicate the receive electric intensity and control the variable attenuator 103. The variable attenuator 103 controls the magnitude of attenuation to the amplified signal so that the input signal level of the frequency converter 104 falls within a predetermined range. The local oscillator 106 oscillates a local oscillation signal LO, which is applied to the frequency converter 104 and is mixed with the amplified high-frequency signal. An intermediate frequency signal IF thus obtained is then applied to the next stage (not shown) at which an intermediate frequency amplifier is located, for example.
A block of a one-dot chained line shown in FIG. 1 denotes an MMIC (Monolithic Microwave Integrated Circuit). For example, a configuration shown in FIG. 2A is used to form the receive front end used in the milliwave band. The configuration shown in FIG. 2A consists of a low-noise amplifier 112 and a frequency converter 114. The low-noise amplifier 112 amplifies a high-frequency signal RF, which is applied to the frequency converter 114 receiving, on the other side, the local oscillation signal LO generated by the local oscillator (not shown). Then, the frequency converter 114 generates the intermediate frequency signal IF from the two received signals. It is not easy to control the signal levels in the milliwave band. Hence, the variable attenuator 103 employed in the configuration shown in FIG. 1 is not used in the configuration shown in FIG. 2A.
A configuration shown in FIG. 2B is used to form the receive front end in the microwave band. The configuration shown in FIG. 2B includes a low-noise amplifier 122, a variable gain amplifier 123, a frequency converter 124, a level detection diode 125. These components are formed by an MMIC.
The high-frequency signal RF is amplified by the low-noise amplifier 122. The amplified output signal is applied to the variable gain amplifier 123, which controls the gain to obtain the predetermined signal level. The output signal of the variable gain amplifier 123 is then applied to the frequency converter 124, which receives the local oscillation signal LO from the local oscillator (not shown). The frequency converter 124 mixes the RF signal from the variable gain attenuator 123 with the local oscillation signal LO, and thus generates the intermediate frequency signal IF. The level detection diode 125 detects the amplified output signal of the variable gain amplifier 123 and thus generates the level detection signal PD.
Generally, the variable gain amplifier 123 is formed, in the microwave band, of a field-effect transistor having a dual-gate structure. The gain control is realized by adjusting the bias applied to a gain control gate terminal of the field-effect transistor.
The receive front end is the important part which determines the receive performance and is required to have a reduced noise level. Hence, the low-noise, high-gain amplifiers 102, 112 and 122 are used. If the amplifiers 102, 112 and 122 output amplified signals having relatively large amplified levels, the frequency converters 104, 114 and 24 of the next stage will receive excessively high levels. Hence, the intermediate frequency signals IF are saturated. With the above in mind, the variable attenuator 103 and the variable gain amplifier 123 are employed.
In the microwave band, the variable gain amplifier 123 can be formed of the field-effect transistor of the dual-gate structure, as described above. However, the dual-gate structure will form a parasitic element in the milliwave band, and thus the variable gain amplifier 123 cannot be employed in the milliwave band. For the above reason, the low-noise amplifier 112 and the frequency converter 114 are formed by the MMIC, as has been described previously.
It is also desired to extend the dynamic range of the receive signal level. It is thus required that the MMIC receive front end used in the milliwave band can vary the signal level at the front end. It is very difficult to sufficiently extend the dynamic range by using the dual-gate field-effect transistor forming the variable gain amplifier 123 in the microwave band.